The propellant tank isolation valves are located between the
propellant tanks and the manifold isolation valves and are used
to isolate the propellant tanks from the remainder of the propellant
distribution system. The isolation valves are ac-motor-operated
and consist of a lift-off ball flow control device and an actuator
assembly that contains a motor, gear train and sector gear. One
pair of valves (one fuel valve and one oxidizer valve) isolates
the propellant tanks from the 1/2 manifold in the forward and
aft left and right RCS. One pair of valves isolates the propellant
tanks from the 3/4/5 manifold in the forward RCS; and two pairs
of valves, in parallel, identified as A and B, isolate the propellant
tanks from the 3/4/5 manifold in the aft left and right RCS.

The forward RCS tank isolation valves are controlled by the fwd
RCS tank isolation 1/2 and 3/4/5 switches on panel O8. The aft
RCS tank isolation valves are controlled by the aft left RCS tank
isolation 1/2 and 3/4/5 A and B and aft right RCS tank isolation
1/2 and 3/4/5 A and B switches on panel O7. These are permanent-position
switches that have three settings: open, GPC and close.

When the fwd RCS tank isolation 1/2 and 3/4/5 switches are positioned
to GPC, that pair of valves is automatically opened or closed
upon command from the orbiter computer. When the corresponding
pair of valves is opened, fuel and oxidizer from the propellant
tanks are allowed to flow to the corresponding manifold isolation
valves. Electrical power is provided to an electrical motor controller
assembly that supplies power to the ac-motor-operated valve actuators.
Once the valve is in the commanded position, logic in the motor
controller assembly removes power from the actuator.

A talkback indicator above each tank's isolation switch on panel
O8 shows the status of that pair of valves. The talkback indicator
is controlled by microswitches in each pair of valves. The talkback
indicator shows op or cl when that pair of valves is open or closed
and barberpole when the valves are in transit or one valve is
open and the other is closed. The open and close positions of
the fwd RCS tank isolation 1/2 and 3/4/5 switches on panel O8
permit manual control of the corresponding pair of valves.

The forward RCS manifold isolation valves are between the tank
isolation valves and the forward RCS engines. The manifold isolation
valves for manifolds 1, 2, 3 and 4 are the same type of ac-motor-operated
valves as the propellant tank isolation valves and are controlled
by the same type of motor-switching logic. The forward RCS manifold
valve pairs are controlled by the fwd RCS manifold isolation 1,
2, 3, 4 and 5 switches on panel O8. When a switch is positioned
to GPC , that pair of valves is automatically opened or closed
upon command from the orbiter computer. A talkback indicator above
the 1, 2, 3, 4 and 5 switch on panel O8 indicates the status of
that pair of valves. The talkback indicator is controlled in the
same manner as the tank isolation valve indication. The open and
close positions of the manifold isolation 1, 2, 3, 4 or 5 switch
on panel O8 permit manual control of the corresponding pair of
valves. The fwd RCS manifold 1, 2, 3 and 4 switches control propellants
for the forward primary RCS engine only.

The fwd RCS manifold 5 switch controls the manifold 5 fuel and
oxidizer valves, which control propellants for the forward vernier
RCS engines only. The switch is normally in the GPC position,
but it can be placed in either open or close for manual override
capability. Electrical power is momentarily applied through logic
in an electrical load controller assembly to energize the solenoid
valves open and magnetically latch the valves. To close the valves,
electrical power is momentarily applied to energize the solenoids
surrounding the magnetic latches of the valves, which allows spring
and propellant pressure to force the valves closed. A position
microswitch in each valve indicates valve position to an electrical
controller assembly and controls a position talkback indicator
above the switch on panel O8. When both valves are open, the indicator
shows op ; and when both valves are closed, it indicates cl .
If one valve is open and the other is closed, the talkback indicator
shows barberpole.

The open, GPC and close positions of the aft left RCS tank isolation
1/2 and 3/4/5 A and B and aft right RCS tank isolation 1/2 and
3/4/5 A and B switches on panel O7 are the same type as those
of the forward RCS tank isolation switches and are controlled
electrically in the same manner. A talkback indicator above each
switch indicates the position of the pair of valves as in the
forward RCS. The 3/4/5 A and B switches control parallel fuel
and oxidizer tank isolation valves to permit or isolate propellants
to the respective aft left and aft right RCS manifold isolation
valves 3, 4 and 5.

The aft left and aft right manifold isolation valves are controlled
by the aft left RCS manifold isolation 1, 2, 3, 4, 5 and aft right
RCS manifold isolation switches on panel O7. The open, GPC and
close positions of each switch are the same type as the forward
RCS manifold isolation switch positions and are controlled electrically
in the same manner. The aft left and aft right RCS manifold 1,
2, 3 and 4 switches provide corre sponding tank propellants to
the applicable primary RCS engines or isolate the propellants
from the engines. The aft left and aft right RCS manifold 5 switch
provides corresponding tank propellants to the applicable vernier
RCS engines or isolates the propellants from the engines.

Each RCS engine is identified by the propellant manifold that
supplies the engine and by the direction of the engine plume.
The first identifier is a letter-F, L or R. These designate an
engine as forward, left aft or right aft RCS. The second identifier
is a number-1 through 5. These designate the propellant manifold.
The third identifier is a letter- A (aft), F (forward), L (left),
R (right), U (up), D (down). These designate the direction of
the engine plume. For example, engines F2U, F3U and F1U are forward
RCS engines receiving propellants from forward RCS manifolds 2,
3 and 1, respectively; the engine plume direction is up.

If either aft RCS pod's propellant system must be isolated from
its RCS jets, the other aft RCS propellant system can be configured
to crossfeed propellant. The aft RCS crossfeed valves that tie
the crossfeed manifold into the propellant distribution lines
below the tank isolation valves can be configured so that one
aft RCS propellant system can feed both left and right RCS engines.
The aft RCS crossfeed valves are ac-motor-operated valve actuators
and identical in design and operation to the propellant tank isolation
valves. The aft RCS crossfeed valves are controlled by the aft
left and aft right RCS crossfeed 1/2 and 3/4/5 switches on panel
O7. The positions of the four switches are open, GPC and close.
The GPC position allows the orbiter computer to automatically
control the crossfeed valves, and the open and close positions
enable manual control. The open position of the aft left RCS crossfeed
1/2 and 3/4/5 switches permits the aft left RCS to supply propellants
to the aft right RCS crossfeed valves, which must be opened by
placing the aft right RCS crossfeed 1/2 and 3/4/5 switches to
the open position for propellant flow to the aft right RCS engines.
(Note that the aft right RCS tank isolation 1/2 and 3/4/5 A and
B valves must be closed.) The close position of the aft left and
aft right RCS crossfeed 1/2 and 3/4/5 switches isolates the crossfeed
capability. The crossfeed of the aft right RCS to the left RCS
would be accomplished by positioning the aft right and left RCS
crossfeed switches to open and positioning the aft left RCS tank
isolation 1/2 and 3/4/5 A, B switches to close . (Note that the
aft left RCS tank isolation 1/2 and 3/4/5 A and B valves must
be closed.)

There are 64 ac-motor-operated valves in the OMS/RCS nitrogen
tetroxide and monomethyl hydrazine propellant systems. Each of
these valves was modified to incorporate a 0.25-inch-diameter
stainless steel sniff line from each valve actuator to the mold
line of the orbiter. The sniff line from each valve actuator permits
the monitoring of nitrogen tetroxide or monomethyl hydrazine in
the electrical portion of each valve actuator during ground operations.

The sniff lines from each of the 12 forward RCS valve actuators
are routed to the respective forward RCS nitrogen tetroxide or
monomethyl hydrazine servicing panels (six to the nitrogen tetroxide
servicing panel and six to the monomethyl hydrazine servicing
panel). The remaining 52 sniff lines are in the left and right
OMS/RCS pods. During ground operations, an interscan checks for
the presence of nitrogen tetroxide or monomethyl hydrazine in
the electrical portion of the valve actuators.

An electrical microswitch located in each of the ac-motor-operated
valve actuators provides an electrical signal (open or closed)
to the onboard flight crew displays and controls and to telemetry.
An extensive program was implemented to reduce the probability
of floating particulates in the electrical microswitch portion
of each ac-motor-operated valve actuator, which could affect the
operation of the microswitch in each valve.